Taliban insurgents in Afghanistan on Saturday announced the start of their annual "spring offensive" against the U.S.-backed government, vowing a nationwide series of attacks as foreign troops withdraw.

Taliban insurgents in Afghanistan on Saturday announced the start of their annual "spring offensive" against the U.S.-backed government, vowing a nationwide series of attacks as foreign troops withdraw.

The extremists said that multiple suicide bombings, "insider attacks" by Afghan soldiers and "special military tactics" would target international airbases and diplomatic buildings to inflict maximum casualties.

They warned Afghans working for President Hamid Karzai's regime to distance themselves from the government to avoid being caught up in the promised violence, and called for young people not to join the police or army.

Afghanistan's fighting season traditionally begins in April or May as snow recedes from the mountains, and in recent years the Taliban have marked the occasion with a public declaration of their intent to bring down Karzai.

Taliban’s latest statement celebrated the start of the NATO withdrawal, saying that "the enemy, with all its military might, has been overwhelmed and finally forced to flee from their military bases".

NATO combat operations in Afghanistan are due to end next year, and coalition commanders claimed that the local army and police have made enough progress to provide security and keep the Taliban at bay.

The United Nations has separately reported a rise of almost 30 percent in civilian casualties in the first quarter compared with the same period last year, with 475 civilians killed and 872 wounded.